[mythtv-users] Is there a distributed filesystem available?

[drescher0110-mythtv at yahoo.com]

> > Expand a raid 5, let's say 20 hours to restripe > just put in a new disk.
> Yup. Agree with this one completely. And it's even worse 
> than that. Last time I looked at this, restriping was error 
> prone (ie not actually officially supported) and required 
> the md device to be offline.
> 
I would avoid this under most situations but the latest kernel 2.6.17 finally has
support for this so this process will get more reliable... See raid5-reshape

> 
> > FS corruption on a raid5, all your data gone, system doesn't work
> I have never seen FS corruption (you're talking about 
> metadata here) using any sensible transactional FS (XFS is 
> great in this respect)
> 
I have not seen this but I admit this is a possibility.

> 
> > 2+ disks dead on a raid5, all your data gone, system doesn't work.
> Yes, this is potentially an issue - But not one that's 
> unavoidable. You have a hotspare, ready in case of any disk 
> failure. This means you waste 2 disks rather than just one.
> 
In this case (as in all new server builds at work) I use raid6 to avoid this
problem. People are going to yell that this is slower yet but if you have a recent
processor you will not notice any large performance problems. I have just set up an
athlon 64 3200 m2 system with a 6 seagate 320GB sata 7200.10 disks in raid6 and with
this system I get over 200MB/s reads and writes which is I do not consider slow.

> If it's a temporary disk failure (connector, psu fault, 
> etc), it is possible to rewrite the RAID superblock to 
> recover the array, since as soon as the array becomes 
> invalid, the kernel will no longer write to the data portion 
> of the array. Of course, this means that you *need* to store 
> your RAID configuration file somewhere apart from on the RAID ;)
> 
> There are further issues with RAID5 - It's write speed is 
> not stellar. To give an idea of the difference, with the 
> same hardware (8 disk software RAID) doing linear writes I 
> have seen:
> 
> 162Mb/sec from RAID5
> 220Mb/sec from RAID0
> 
Compair this to single drive speed which is what the original poster of this thread
was asking for...

John